Filter cloth and method for producing the same

ABSTRACT

The invention relates to a filter cloth for a filter press and to a method for producing said filter cloth. The inventive filter cloth has a filtration section through which the filtrate reaches a filter element of the filter press, and an edging section which rests against the edge of the filter element. The filtration area is provided with a cloth that differs from that of the edging section. The filtration area is woven from monofil yarns while multifil yarns are woven into the edging section.

[0001] The invention relates to a filter cloth for a filter press,having a filtration area through which filtrate can pass into a filterelement of the filter press, and a border area which is provided forapplication to an edge of the filter element. The invention also relatesto a method for the manufacture of the filter cloth for a filter press.

[0002] A filter press is a discontinuously operating filtration device,which is more particularly used in the raw material, foodstuffs,pharmaceutical and chemical industries. The filter press generallycomprises a large number of parallel-linked, plate-like filter elements,between which are located filter cloths. The filter elements may notonly be plate-like, but can also have a different shape. The faces ofthe plate group are bounded by a fixed wall or cover plate and a movablepressure plate. The pressure plate presses together the plate-likefilter elements, which are made from metal, plastic or some othersuitable material and as a result the filter press is closed.Subsequently a suspension to be filtered is introduced into the filterpress, filtrate passing through the filter cloth whilst forming a filtercake thereon and is led off via corresponding channels on the plate-likefilter element. For removing the filter cake formed, the axial tensileforce is relieved and the plate-like filter elements separated. Thefilter cake formed on the individual filter cloths is ejected or isscraped off by means of a doctor blade.

[0003] As a result of the method the axial force for pressing togetherthe filter elements must be so high that it significantly exceeds thefilling pressure of the suspension to be filtered. The filter cloth usedfor a filter press not only serves as a filtering medium, but also as aseal along the filter element edge, where the adjacent filter elementsare pressed against one another. To ensure that this sealing function isadequately performed, particularly at the contact surface between thefilter element edge and the filter cloth, said filter cloth must have anadequate deformability, so that liquid passages are prevented.

[0004] However, in the vicinity of the filtration surface of the filtercloth a very dimensionally stable, robust filter cloth structure isdesired.

[0005] It is known to weave a filter cloth from monofilament yarns,which are also known as plastic wires. However, a fabric formed frommonofilament yarns is unsuitable for sealing the edges of the filterelements, because the wire-like yarns do not adequately elasticallydeform under the axial contact pressure and tend to break duringprolonged operational loads.

[0006] Therefore multifilament yarn fabrics are used for filter clothsfor filter presses. Such multifilament yarns, which is also understoodto cover hereinafter staple fibre yarns, comprise a plurality of fibres,which leads to an improved deformability under pressure and therefore adesired sealing action. However, a disadvantage of such multifilamentyarn fabrics is that in the filtration area solid attachments formbetween the individual fibres of a multifilament yarn and such fabricsmat together and/or clog after a relatively short operating period.Thus, multifilament yarn filter cloths when used on a filter pressrequire relatively frequent replacement or cleaning, which reduces thedegree of utilization of a filter press and leads to corresponding costswhen replacing said filter cloths.

[0007] The object of the invention is to provide a filter cloth, whichpermits a particularly economic operation of a filter press. A furtherobject is to provide an economic method for the manufacture of such afilter cloth.

[0008] The first part of the object is achieved by a filter cloth havingthe features of claim 1. According to the invention, the second part ofthe object is achieved by a method having the features of claim 8.Preferred embodiments of the invention are given in the dependentclaims.

[0009] The filter cloth according to the invention is characterized inthat the filtration area has a fabric differing from that of the borderarea, that the filtration area is preponderantly woven from monofilamentyarns and that preponderantly multifilament yarns are woven in theborder area.

[0010] In this way, in the case of the filter cloth according to theinvention, it is possible to combine the advantages of a monofilamentfabric and a multifilament fabric. Through the use of monofilament yarnsin the filtration area it is possible to ensure a particularly robustand consequently long-life filtering medium, even under relatively roughoperating conditions in a filter press. As a result of the smooth yarnsurface of the monofilament yarn a reliable and substantially completedischarge of the filter cake from the filtration area is ensured. Inaddition, the monofilament fabric ensures a high shear strength andexcellent dimensional stability. The use of multifilament yarns in theborder area, which engages on the projecting edge of the filter element,ensures an adequate deformability, which is essential for a good sealingfunction. Whereas in the case of multifilament filter cloths of aconventional nature, through having to take account of the sealingfunction, the filter cloth openings are relatively fine throughout, thefilter cloth according to the invention permits a significant freedom ofdesign with regard to the size of the openings in the filtration area.The openings preferably have a size between 10 and 100 um. The filtercloth can also be used in other filters, in which there is a similarproblem of a simultaneous filtering and sealing function.

[0011] To a certain extent there can also be multifilament yarns in thefiltration area. A particularly robust, long-life filter cloth for afilter press is obtained according to the invention in that thefiltration area is woven exclusively from monofilament yarns.

[0012] It is also possible to provide only multifilament yarns in theborder area. According to a further development of the invention, in theborder area are woven both multifilament yarns and monofilament yarns.Through the combination of the two yarns in the border area it isensured that on the one hand an adequate deformability and elasticity isensured through the multifilament yarns and on the other themonofilament yarns ensure an adequate stability against tearing.Preferably preponderantly multifilament yarns are used in the borderarea to ensure a good sealing function.

[0013] To ensure an adequate sealing function in the aforementionedembodiment, it is preferable for the multifilament yarns in the borderarea to be located preponderantly on the side of the filter cloth facingthe filter element. Thus, the contact surface between the edge of thefilter element and the filter cloth being supported is particularlyproblematical with regards to sealing. On pressing together two facingfilter elements with filter cloths spread out thereon, the contactsurface between the individual filter cloths is scarcely problematical,because even on pressing together two monofilament fabrics an adequateseal against the passage of liquid is ensured. Conversely in thefiltration area the monofilament yarns are preponderantly on the filtercake side in order to ensure a good and substantially complete dischargeof the filter cake.

[0014] According to a further development of the invention the filtercloth is constructed as a double layer fabric, a lower fabric layerfacing the filter element is a backing fabric and an upper fabric layeris constructed as a filtration-active layer. The upper fabric layer canbe adjusted in such a way that there is a desired opening size for thepassage of filtrate. For this purpose relatively fine yarns can be used,whereas the lower fabric layer can be made from a wide meshed fabricwith relatively thick yarns.

[0015] According to the invention the use time of the filter cloth canbe increased in that a border area located in the discharge direction ofa filter cake formed is provided on its top surface remote from thefilter element preponderantly or exclusively with monofilament yarns.During filter cake discharge over the marginal area, the problem arisesthat filter cake residues can stick to the filter cloth in said marginalarea. On closing the filter press again, these residues constitute avery considerable burden for the filter cloth, so that such residuesfrequently cause damage to the filter cloth. A fabric of monofilamentyarns with smooth top surface ensures a reliable sliding off of thefilter cake and consequently reduces the filter cloth damage risk. Thefilter cloth can be produced with an angular, round or other shape. Forso-called tower filter presses with progressively circulating filtercloth belt, the filter cloth can be constructed as an annular, closedbelt with a plurality of transverse borders.

[0016] The method according to the invention for the manufacture of afilter cloth for a filter press is characterized in that the filtrationarea is woven differently from the border area, that the filtration areais preponderantly woven from monofilament yarns and that in the borderarea preponderantly multifilament yarns are woven. In this way theaforementioned filter cloth according to the invention can beeconomically manufactured.

[0017] According to a further development of this method, it isparticularly economically advantageous for the filter cloth to be madeup from a length of fabric and for the latter to be produced in a widthcorresponding to a desired side length of the filter cloth. The filtercloth for a filter press with predetermined filter element dimensionscan in this way be woven and finished, e.g. heat treated and/orstretched and can be produced by simply cutting to length the length offabric.

[0018] Preferably, along the marginal edges of the length of fabric andcorresponding to the desired border area of the filter cloth,multifilament yarns are used as warp threads. As opposed to this, forforming the filtration area monofilament yarns are used here as warpthreads. In these areas monofilament yarns are also used as weftthreads.

[0019] According to another embodiment of the method according to theinvention, corresponding to the other side length of the quadrangularfilter cloth in transversely directed portions of the length of fabricmultifilament yarns are used as weft threads for forming the borderarea. This method permits simple manufacture of the filter clothaccording to the invention with the specifically constructed filterareas and border areas by weaving, finishing and making up.

[0020] The invention is further illustrated hereinafter by preferredembodiments described in greater detail relative to the attacheddrawings, wherein show:

[0021]FIG. 1 A part cross-sectional view through a filter press duringfiltration.

[0022]FIG. 2 A perspective view of a filter press with separated filterelements.

[0023]FIG. 3 A plan view of the length of fabric for producing a filtercloth according to the invention.

[0024]FIG. 4 A part cross-sectional view through a fabric of a filtercloth according to the invention.

[0025] The basic operating principle of a filter press 50 and thearrangement of the filter cloth 10 according to the invention isdiagrammatically illustrated by FIGS. 1 and 2. The filter press 50comprises several plate-like filter elements 52, which have a centralfiltration portion provided with outflow channels and surrounded by aframe-like, projecting edge 54. The arrangement of the outflow channelsand the edge 54 on the filter elements 52 is bilateral. A filter cloth10 is placed on both sides of the filter element 52 and a one-piecefilter cloth 10 can be used by turning over on one side of the filterelement 52.

[0026] For filtration operation the filter press 50 is closed, in thatthe parallel filter elements 52 are pressed axially against one another.The edges 54 of the individual filter elements 52 are adjacent to oneanother, the filter cloth 10 resting on the edge 54 sealing the contactsurface. In accordance with arrow P1 a suspension to be filtered can beintroduced under pressure into the cavity between the filter elements52. Accompanied by the formation of a filter cake at the filtration area12 of the filter cloth 10, the filtrate penetrates through the filtercloth 10 and runs out through the outflow channels in the filterelements 52 and central channels in accordance with the arrow P2. Onreaching a given filter cake height the filter press 50 is opened inaccordance with FIG. 2, the filter cake formed on the filter cloth 10dropping downwards or being scraped off.

[0027] To comply with the different conditions during filtrationoperation, the border area 14 of the filter cloth 10 according to theinvention and which rests on the raised edge 54 of the filter element52, has a fabric formed preponderantly from multifilament yarns.However, in its filtration area 12, the so-called filtration window, thefilter cloth 10 has a fabric formed preponderantly from monofilamentyarns.

[0028] The manufacture of a filter cloth 10 according to the inventionwill be explained in conjunction with FIG. 3. The filter cloth 10 ismade up from a length of fabric 40 produced by a conventional weavingmethod by weaving longitudinally directed warp threads with transverselydirected weft threads. The length of fabric 40 is woven and finished inaccordance with a desired width of the filter cloth 10 and in accordancewith the desired border area 14 along the marginal edges 42 monofilamentyarns are used as warp threads. In a transversely directed portion 44,which is also intended to serve as a border area 14 as so-calledtransverse borders of the filter cloth 10 to be made up, additionallymultifilament yarns are used as weft threads. The width of thetransversely directed portion 44 is designed for a turn round the edge54 of the filter element 52 or roughly twice as large as the desiredwidth of the border area 14. In the latter case cutting of the filtercloth 10 from the length of fabric 40 would take place in the centre ofportion 44, so that the filter cloths 10 can be made up without wastefrom the length of fabric 40. In other areas of the length of fabric 40mainly monofilament yarns are used both as warp threads and weftthreads.

[0029]FIG. 4 is a part cross-sectional view through a filter cloth 10according to the invention with a double layer fabric. Said double layerfabric comprises a lower backing fabric 22 facing the filter element,whilst on the so-called filter cake or satin side there is a finermeshed fabric as filtration-active layer 24. In all there are an upperweft thread 26, a central binding thread 28 and a lower weft thread,which are woven with fine, upper warp threads 32 and lower, larger warpthreads 34.

[0030] The upper weft threads 26 and the upper, fine warp threads 32form the filtration-active layer 24, whereas the upper weft thread 26 isalways a monofilament yarn. Corresponding to the arrangement in a borderarea 14 or a filtration area 12, the upper warp threads 32 can bemultifilament or monofilament yarns.

[0031] The lower backing fabric 22 is formed by the lower warp threads34 and the lower weft threads 30 and said threads can be multifilamentyarns in a border area 14. The central binding threads 28 serve toprovide a reliable connection of the filtration-active layer 24 with theunderlying backing fabric 22.

1. Filter cloth for a filter press (50), having a filtration area (12)through which filtrate can pass into a filter element (52) of the filterpress (50), and a border area (14) which is provided for application toan edge (54) of the filter element (52), characterized in that thefiltration area (12) has a fabric differing from that of the border area(14), that the filtration area (12) is preponderantly woven frommonofilament yarns and that preponderantly multifilament yarns are wovenin the border area (14).
 2. Filter cloth according to claim 1,characterized in that the filtration area (12) is exclusively woven frommonofilament yarns.
 3. Filter cloth according to claim 1 or 2,characterized in that in the border area (14) are woven bothmultifilament yarns and monofilament yarns.
 4. Filter cloth according toclaim 3, characterized in that in the border area (14) the multifilamentyarns are preponderantly located on the side of the filter cloth (10)facing the filter element (52).
 5. Filter cloth according to one of theclaims 1 to 4, characterized in that it is constructed as a double layerfabric, that a lower fabric layer facing the filter element (52) is abacking fabric (22) and that an upper fabric layer is constructed as afiltration-active layer (24).
 6. Filter cloth according to one of theclaims 1 to 5, characterized in that a border area (14) located in thedischarge direction of a filter cake formed has on its top side remotefrom the filter element (52) preponderantly monofilament yarns. 7.Filter cloth according to one of the claims 1 to 6, characterized inthat in the filtration area (12) the monofilament yarns arepreponderantly located on the side of the filter cake (10) which isremote from the filter element (52).
 8. Method for the manufacture of afilter cloth (10) for a filter press (50) according to one of the claims1 to 7, characterized in that a filtration area (12) is wovendifferently from the border area (14), that the filtration area (12) ispreponderantly woven from monofilament yarns and that in the border area(14) preponderantly multifilament yarns are woven.
 9. Method accordingto claim 8, characterized in that the filter cloth (10) is manufacturedfrom a length of fabric (40), which is produced with a widthcorresponding to a desired side length of the filter cloth (10). 10.Method according to claim 9, characterized in that along the marginaledge (42) of the length of fabric (40) and corresponding to the desiredborder area (14) of the filter cloth (10), multifilament yarns are usedas warp threads.
 11. Method according to claim 10, characterized in thatin accordance with the other side length of the rectangular filter cloth(10) in transversely directed portions (44) of the length of fabric (40)use is made of multifilament yarns as weft threads for forming theborder area (14).